geostatistical approach to estimating rainfall over mauritius mphil/phd student: mr.dhurmea k. ram...

Geostatistical approach to Estimating Rainfall over Mauritius

Mphil/PhD Student:Mr.Dhurmea K. Ram

Supervisors:Prof. SDDV Rughooputh Dr. R Boojhawon

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Objectives of Research

Analyse qualitatively and quantitatively the spatial distribution of rainfall over Mauritius.

Develop models for predicting values of rainfall at other locations from available measured rainfall values using geostatistics coupled with regression models.

Generate digital results that can eventually be used in Geographical Information Systems (GIS).

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Importance of modellingPrecipitation measurements are

unevenly distributed over a region and even less in a mountainous areas.

One of the fundamental problem in hydrology is to estimate precipitation at unmonitored site using data from available surrounding precipitation stations.

Hydrological models require precipitation fields on grid systems and in digital form.

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Background work Study particularly aim at modelling the long-

term variations in the distribution where the monthly mean values of precipitation for the period 1971-2000 are analysed for each month, i.e. for January till December using geostatistics.

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• Rainfall is also affected by elevation of an area and also by prevailing atmospheric conditions•Rainfall is such a variable whose structure depends on direction (anisotropic).•Need for another method whereby this anisotropic behavior can be modeled.•Most common technique is geostatistics•Special branch of statistics developed by George Matheron (1963) (Centre de Morphologie Mathematique).

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•It is the principle tool of geostatistics•The variogram is a measure of how quickly things change on the average. •The variogram gives an insight of the geometry and continuity of the variable •The variogram is a function of direction, i.e. it characterizes the dependence that exist between variables at different points in space •The underlying principle : on the average, two observations closer together are more similar than two observations farther apart.

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Exploratory data analysis

Analyze data for trend, outliers and skewness.

Outliers observed for most of the months.No significant trend over short interval

but trend exist over long period. Data found to be skewed. Square root and

log transformation applied to normalize data.

Modelling is carried out using raw data, outlier removed data and detrended and transformed data

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Model ApproachesBasic geostatisitics model:

Raw data Outlier removed Transformed data

Detrended data model: Raw data Outlier removed

Geostatistical- topographical model

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Results (1)More accurate estimates obtained from

detrended data model.Better estimates for winter months.High error values limited to less than 25 % of

the stations. Models yield very good estimates for 75 % of stations.

No improvements in estimates when removing outliers.

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Results (2)Comparing with the rainfall volume

calculated at the Mauritius Meteorological Services shows that actually the long term mean monthly rainfall, for 1971-2000, has been underestimated for most of the months using the isohyetal analysis.

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Challenges

Optimize variograms particularly for summer months to improve accuracy of models

Derive values of humidity, wind speed and direction, pressure and temperature on a regular grid from few available stations.

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Limitations

Very few stations on mountainous areas. Availability of finer resolution Digital

Elevation Model .Effective estimation of missing rainfall data.

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Future Works (1)To develop individual model templates for different weather systems.

Include atmospheric variables such as pressure, temperature, humidity and wind speed and directions as additional parameters in the models.

Besides using Ordinary Kriging and regression models, use cokriging to incorporate the secondary variables.

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Future Works (2)Model using a smaller number of

stations. This aims at removing redundant, rain gauges and their relocation.

Implement precipitation forecasting models using artificial neural networks techniques.

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ConclusionsGeostatistics turned is a very good

approach for estimating rainfall.Accuracy of model estimates depends

on degree of optimisation of variograms.

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AcknowledgementsUniversity of Mauritius for providing logistic

support.Mauritius Research Council for funding this

study.Mauritius Meteorological Services for

providing required data.

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